can static method overridden?

[derek_ho]

Is this true:
"static methods can be overridden by static methods only".( from java_cert exam)
The answer is false. can someone tell me why?
can static method be overriden? If yes, what are ways of overriding a static method?

[Rob Ross]

Originally posted by derek_ho:
Is this true:
"static methods can be overridden by static methods only".( from java_cert exam)
The answer is false. can someone tell me why?
can static method be overriden? If yes, what are ways of overriding a static method?

No, it is not true. Static methods cannot be overridden, rather, they can be hidden.
This program shows the difference between method hiding and overridding.

public class StaticHiding
{
    static public void main(String[] args)
    {
        SubClass object = new SubClass();//concrete object is of type SubClass
        //Creates two new references that point to the existing SubClass object
        SubClass   subClassReference = object;
        SuperClass superClassReference = object;
        //invoke an overriden instance method on both references
        //these are  polymorphic method calls!
        subClassReference.printClassName();
        superClassReference.printClassName(); //line 1
        System.out.println();
        //invoke a class method on each reference
        subClassReference.staticPrintClassName();
        superClassReference.staticPrintClassName(); //line 2
        //line 2 does not call the overriden version!!!!!
    }
}
class SuperClass
{
    public void printClassName()
    {
         System.out.println("I am a SuperClass method");
    }
    static public void staticPrintClassName()
    {
           System.out.println("I am a SuperClass static method");
    }
}//end class SuperClass
class SubClass extends SuperClass
{
    /** This method overrides the inherited version from the SuperClass
     */
    public void printClassName()
    {
         System.out.println("I am a SubClass method");
    }
    //This method declaration DOES NOT OVERRIDE THE SUPERCLASS METHOD!
    static public void staticPrintClassName()
    {
           System.out.println("I am a SubClass static method");
    }
}//end class SubClass extends SuperClass

As you can see, line 1 works as expected. You are making a method call on an SuperClass object reference variable, but because the actual instance is of type SubClass, the overridden method gets called.
In line 2, when we attempt to do the same thing with a static method, we see that the same rules do not apply. The static method that gets invoked is the one that is a member of the reference variable's class, NOT the one that is a member of the actual object instance!
This shows that there is a very real, tangible, important distinction between static and instance methods, and between hiding and overridding. It also shows why it is Bad Form to refer to a static member by an reference variable; you should always use the class name to access a static member. In the above line 2, if you had written
SubClass.staticPrintClassName();
SuperClass.staticPrintClassName();
it would be clear which method would be invoked.

Rob

[Valentin Crettaz]

See also the following thread for a good discussion about the subject:
http://www.javaranch.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=24&t=014224
HIH

[Seany Iris]

Rob,Based on the code,how to invoke the method printClassName() of superClass?

[Rob Ross]

Seany,
there are only two ways.
One, instantiate an object of type SuperClass and call that method.
SuperClass sc = new SuperClass();
sc.printClassName();
Two, create a subclass of SuperClass. Subclasses can call super.printClassName() and access the overridden method.

Rob

[Seany Iris]

Two, create a subclass of SuperClass. Subclasses can call super.printClassName() and access the overridden method

Rob, thank you.
I know the first way you told.
But as for the second,It's only be called in subclass? If out the subClass, can it?

[Rob Ross]

No Seany. If you have an object instance of type SubClass, and you call printClassName()on that object, there is no way for you to invoke the SuperClass version. The only method available to you is the overridden version.
Rob

[Seany Iris]

I see,thank you very much,Rob!